1. Field of the Invention
The present invention concerns a magnetic resonance system with a transmission antenna and a radio-frequency source having a sheath wave barrier or trep in a cable between the radio-frequency source and the examination volume.
2. Description of the Prior Art
PCT Application WO 03/025608 discloses a magnetic resonance system wherein an examination subject arranged in an examination volume can be excited to magnetic resonance with an excitation frequency by means of a transmission antenna, operated by a radio-frequency source, with the radio-frequency source having stationary or mobile in the examination volume or in its surroundings.
An electrical field oscillating at the excitation frequency is generated by the radio-frequency source.
A barrier is arranged between the radio-frequency source and the examination volume, the barrier being formed of individual resonators that are each naturally resonant at the excitation frequency.
The examination volume is shielded by means of the barrier from the electrical field generated by the radio-frequency source.
The radio-frequency source is fashioned as a cable in this known magnetic resonance system. The barrier essentially represents a further development of a known sheath wave barrier.
The transmission antennas of a magnetic resonance system should generate a magnetic excitation field in an examination subject (generally a person) to cause magnetic resonance excitatism in the examination subject. After the magnetic resonance excitatism it is possible to receive the emitted magnetic resonances by means of suitable reception antennas. The transmission antenna itself can possibly be used for reception.
Eddy currents, which lead to an unwanted heating of the examination subject, are always associated with the generation of the magnetic excitation field. These eddy currents cannot be prevented. Currents having electrical fields that are capacitively coupled into the examination subject and lead to a further heating of the examination subject are also generated in addition to these unavoidable eddy currents. Such capacitive couplings in particular occur at the conductors of the transmission antenna, but can also occur (albeit to a lesser extent) at local receiver coils or at cables insofar as these are located in the effective range of the transmission antenna. All of these elements (transmission antenna, local coil, cable) can thus act as (unwanted) radio-frequency sources whose electrical fields should optimally be kept away from the examination subject.
A direct and (without further measures) intrusive approach is to make the distance of the radio-frequency source from the examination subject optimally large. This leads, for example, either to over-dimensioned transmission antennas in the case of whole-body transmission antennas, or to a reduction of the spatial relationships (which are limited anyway) in the examination volume. An enlarged distance in the reception case also leads to a reduced sensitivity, even for local coils.
It would also be possible to divide the resonance capacitors in the transmission and/or reception antennas (known as “multiple reduction”). This leads, however, to increased capacitor losses and moreover to an additional production expenditure. Furthermore, this solution is not applicable in all cases.